1. Field of the Invention
The present invention relates to a display apparatus employing pixel circuits each having an electro-optical device. The pixel circuit is also referred to merely as a pixel whereas the electro-optical device is also referred to as a display device or a light emitting device. To put it in more detail, the present invention relates to a display apparatus in which each of the pixel circuits employs an electro-optical device to serve as an electrically driven display device emitting light with a luminance varying in accordance with the magnitude of a driving signal driving the device and employs an active element for controlling the driving signal so that the display apparatus can be driven to display an image in pixel units.
2. Description of the Related Art
There is a display apparatus employing pixel circuits each having an electro-optical device serving as a display device which emits light with a luminance varying in accordance with the magnitude of a driving signal which is a voltage applied to the device or a current flowing through the device. For example, a liquid crystal display device is a typical electro-optical device which emits light with a luminance varying in accordance with the magnitude of a voltage applied to the device. On the other hand, an organic EL (Electro Luminescence) device and an OLED (Organic Light Emitting Diode) are each a typical electro-optical device which emits light with a luminance varying in accordance with the magnitude of a current flowing through the device. A display apparatus employing pixel circuits each including an organic EL device is referred to as an organic EL display apparatus which is a display apparatus of the so-called light-self-emission type. A display apparatus of the light-self-emission type is a display apparatus employing pixel circuits each including an electro-optical device of the light-self-emission type.
The organic EL device has an organic thin film which is also referred to as an organic layer sandwiched by a lower electrode and an upper electrode. The organic layer is a laminated layer composed of an organic hole transport layer and an organic light emitting layer. The organic EL device is an electro-optical device utilizing a phenomenon in which light is emitted when an electric field is applied to the organic thin film. Thus, by controlling a current flowing through the organic EL device, it is possible to obtain gradations of emitted colors.
Since the organic EL device can be driven by a relatively low voltage such as a voltage not exceeding 10 V, the organic EL device has a low power consumption. In addition, since the organic EL device is a light self-emitting device which emits light by itself, the organic EL display apparatus employing pixel circuits each having an organic EL device does not require an auxiliary illumination member such as a backlight which is needed in a liquid-crystal display apparatus. It is thus easy to reduce the weight and thickness of the organic EL display apparatus employing pixel circuits which each have an organic EL device. On top of that, since the response speed of the organic EL device is extremely high, a residual image of a moving-image display is not generated. A typical response time of the organic EL device has a value of the order of several microseconds. Due to the merits offered by the organic EL device as described above, a planar light-self-emission display apparatus, which employs pixel circuits each having an organic EL device to serve as an electro-optical device, is developed intensively and extensively in recent years.
By the way, a display apparatus employing pixel circuits each having an electro-optical device can adopt a simple (passive) matrix method or an active matrix method as a driving method. Representative examples of the display apparatus employing pixel circuits each having an electro-optical device are a liquid-crystal display apparatus employing pixel circuits each having a liquid-crystal display device and an organic EL display apparatus employing pixel circuits each having an organic EL device. However, even though a display apparatus adopting the simple matrix method has a simple structure, such a display apparatus raises a problem that it is difficult to design the display apparatus into a display apparatus having a large size or a high-definition display apparatus.
In order to solve the problem described above, the active matrix method is developed intensively and extensively in recent years. In accordance with the active matrix method, a pixel signal supplied to a light emitting device employed in a pixel circuit is controlled by making use of an active device, which is employed in the same pixel circuit as the light emitting device, as a switching device. A typical example of the active device is an insulated-gate FET (Field Effect Transistor) which is generally a TFT (Thin Film Transistor).
In order to drive an electro-optical device of such a pixel circuit to emit light, an input image signal is supplied to a signal holding capacitor employed in the pixel circuit to serve as a capacitor for holding the input image signal. The input image signal is supplied to the signal holding capacitor employed in the pixel circuit through a video-signal line. In the pixel circuit, the input image signal is supplied from the video-signal line to the gate electrode of a driving transistor for driving the electro-optical device as well as a signal holding capacitor connected to the gate electrode. The input image signal is supplied from the video-signal line to the gate electrode of the driving transistor as well as the signal holding capacitor by way of a switching transistor which is also referred to as a sampling transistor. In this way, a driving signal according to the voltage of the input image signal held in the signal holding capacitor is then supplied to the electro-optical device. It is to be noted that, in the following description, the gate electrode of the driving transistor is also referred to as a control input terminal of the driving transistor whereas the signal holding capacitor is also referred to as a pixel capacitor.
In a liquid-crystal display apparatus employing pixel circuits each having a liquid-crystal display device to serve as an electro-optical device, the liquid-crystal display device is a device of a voltage-driven-emission type. Thus, the liquid-crystal display device is driven by a voltage signal according to the input image signal held in the signal holding capacitor. In an organic EL display apparatus employing pixel circuits each having a current-driven-emission device such as an organic EL device to serve as an electro-optical device, on the other hand, the voltage signal according to the input image signal held in the signal holding capacitor is converted by the driving transistor into a current signal and the organic EL device is driven by the current signal.
Represented by an organic EL device, the electro-optical device of the current-driven-emission type is a device emitting light with a luminance which varies as the current driving the device varies. In order to drive the electro-optical device to emit light with a stable luminance, it is important to supply a stable driving current to the electro-optical device. Methods for supplying a driving current to an organic EL device are classified typically into two large categories, i.e., a constant-current driving method and a constant-voltage driving method. Since each of the constant-current driving method and the constant-voltage driving method is a method based on a known technology, however, the references made available to the public as references disclosing these methods are not given in this patent specification.
Since the voltage-current characteristic of the organic EL device has a large gradient, execution of the constant-voltage driving method gives rise to large current variations due to even small variations in voltage and/or small variations in device characteristic. The large variations in current in turn bring about large variations in luminance. For this reason, it is generally necessary to adopt the constant-current driving method which makes use of the driving transistor in a saturated region. Of course, even in the case of the constant-current driving method, if variations in current exist, such variations will result in variations in luminance. If the variations in current are small, however, the variations in luminance are also small as well.
Conversely, even if the constant-current driving method is adopted, in order to make the luminance of light emitted by the electro-optical device stable, it is important to sustain the driving signal, which is being held in the signal holding capacitor as a signal according to the input image signal, at a fixed magnitude. For example, in order to make the luminance of light emitted by the organic EL device stable, it is important to sustain the driving current, which is flowing through the driving transistor as a current according to the input image signal, at a fixed magnitude.
By the way, variations in manufacturing process inevitably give rise to threshold-voltage and mobility variations of an active device for driving the electro-optical device. In addition, the characteristic of the electro-optical device such as the organic EL device also changes with the lapse of time. These threshold-voltage and mobility variations of the active device for driving the electro-optical device and these changes of the characteristic of the electro-optical device undesirably have an effect on the luminance of light emitted by the electro-optical device.
For the reasons described above, in order to control the luminance of light emitted by the electro-optical device to a value uniform throughout the entire display screen of the display apparatus, there has been studied a variety of mechanisms for compensating each pixel circuit for the threshold-voltage and mobility variations of the active device employed in the pixel circuit to serve as a device for driving the electro-optical device and for the changes of the characteristic of the electro-optical device of the pixel circuit.
For example, a mechanism described in Japanese Patent Laid-Open No. 2007-310311 (hereinafter referred to as Patent Document 1) has three proposed functions, i.e., a threshold-voltage compensation function, a mobility compensation function and a bootstrap function, for a pixel circuit employing an organic EL device. The threshold-voltage compensation function is a function for sustaining the driving current generated by the driving transistor at a constant magnitude, given a constant input image signal, even if the threshold voltage of the driving transistor varies from transistor to transistor and changes with the lapse of time. The mobility compensation function is a function for sustaining the driving current generated by the driving transistor at a constant magnitude, given a constant input image signal, even if the mobility of the driving transistor varies from transistor to transistor and changes with the lapse of time. The bootstrap function is a function for sustaining the driving current generated by the driving transistor at a constant magnitude, given a constant input image signal, even if the current-voltage characteristic of the organic EL device changes with the lapse of time.